1. FIELD OF THE INVENTION
This invention relates to article surveillance systems, and more particularly to a proximate surveillance system having a single coil, coplanar antenna which enhances the sensitivity and reliability of the system.
2. DESCRIPTION OF THE PRIOR ART
Coplanar antenna proximate surveillance systems have been disclosed for use in article surveillance systems. They function to remind a cashier or sales clerk to remove a magnetic marker from a purchased item before it is carried through a theft detecting interrogation zone provided near the egress of an area appointed for surveillance.
Generally, coplanar antenna proximate surveillance systems use separate transmit and receive antennae. A major problem with such systems is the tendency of the receive antenna to generate a magnetic field which opposes the magnetic field generated by the transmit antenna. The presence of the opposing magnetic fields, sometimes referred to as the "transformer effect", arises from the transformer action (magnetic coupling between the transmit and receive antenna coils.
One means for reducing the transformer effect, involves the incorporation of a "figure 8" type transmit antenna with a receive antenna centered thereabout. This means affords some reduction of the transformer effect, but creates a dead zone in the area perpendicular to and near the center of the "figure 8" antenna such that the magnetic marker can pass through the interrogation zone undetected.